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Smallest known galaxy with a supermassive black hole

This Hubble Space telescope image shows the gargantuan galaxy M60 in the center, and the ultracompact dwarf galaxy M60-UCD1 below it and to the right, and also enlarged as an inset. A new international study led by University of Utah astronomer Anil Seth found that M60-UCD1 is the smallest known galaxy with a supermassive black hole at its centre, suggesting the dwarf galaxy originally was much larger but was stripped of its outer layers by gravity from galaxy M60 over billions of years. M60's gravity also is pulling galaxy NGC4647, upper right, and the two eventually will collide.

NASA/SPACE TELESCOPE SCIENCE INSTITUTE/EUROPEAN SPACE AGENCY

Many black holes may hide in dwarf remnants of stripped galaxies, a University of Utah astronomer and his colleagues believe after discovering an ultracompact dwarf galaxy that harbours a supermassive black hole.

If correct, it means black holes may be more common than we thought.

"It is the smallest and lightest object that we know of that has a supermassive black hole," says Anil Seth, lead author of an international study of the dwarf galaxy published today in Nature. "It's also one of the most black hole-dominated galaxies known."

The small galaxy named M60-UCD1 has a black hole with a mass equal to 21 million Suns.

Seth, assistant professor of physics and astronomy at the University of Utah, believes that dwarf galaxies like this may be the stripped remnants of larger galaxies that were torn apart during collisions with yet other galaxies.

"We don't know of any other way you could make a black hole so big in an object this small. There are a lot of similar ultracompact dwarf galaxies, and together they may contain as many supermassive black holes as there are at the centers of normal galaxies."

Black holes are collapsed stars and collections of stars with such strong gravity that even light is pulled into them. Supermassive black holes – those with the mass of at least a million stars like our Sun – are thought to be at the centres of many galaxies.

The central, supermassive black hole at the centre of our Milky Way galaxy – that we discussed in a recent Cosmos issue – has the mass of 4 million Suns. By comparison, the supermassive black hole at the centre of ultracompact dwarf galaxy M60-UCD1 is five times larger than the Milky Way's.

M60-UCD1 is about 54 million light years from Earth, but the dwarf galaxy is only 22,000 light years from the centre of galaxy M60, which "is closer than the sun is to the centre of the Milky Way," Seth says.

The video simulation below, made by one of Seth's co-authors, Holger Baumgardt of the University of Queensland, shows how galaxy M60-UCD1, was formed from a larger, normal galaxy.

The video begins with a background image from the Hubble Space Telescope, with the huge elliptical galaxy M60 in the centre, galaxy NGC4647 in the upper right and MC60-UCD as a small whitish spot lower right.

As the video begins, a normal galaxy (yellow and red) orbits M60. During an estimated 500 million years, M60’s gravity strips stars (red material) from the orbiting galaxy, leaving as a remnant the ultracompact dwarf galaxy now known as M60-UCD1. The end of the video zooms in on the Hubble Space Telescope close-up image of M60-UCD1.

For the first time, astronomers have observed jellyfish galaxies ‘feeding’ supermassive black holes. The discovery is a key clue in the long-standing puzzle to understand how galaxies form and change in our expanding and evolving Universe.